The present invention relates to an image enhancement method and system and more particularly, to an image enhancement method and system for low contrast or stationary images.
The use of video cameras, video movies and video-tapes has, to a large extent, replaced films, as the media for image recording and storage. Consequently, the use of digital storage of video images is growing rapidly and digital storage begins to replace tape-storage.
To digitally store a video image, or a video movie, one usually digitizes the data with a frame grabber. This device digitizes the value of each pixel in the video-image and then stores the digitized data on various types of digital memories, such as a computer memory, hard disk drives or optical disks. The reliability of digital storage devices on the one hand, coupled to the continuous reduction in their cost/performance (optical disks), makes digital storage the method of choice.
Most frame grabbers digitize the image with 8-bit accuracy, i.e., 1 gray level in 256 levels for the entire dynamic range of an image. This digitization is accomplished at video-rates (up to 60 Hz), and required digitization at 10-40 MHz. This accuracy is not sufficient for various scientific and industrial applications, as well as not sufficient for "natural" color display of digital images. One suggested solution is to digitize the image with 12 or more bits. This solution is technically demanding, and a proposed 12-bit frame grabber is even far more expensive than the common 8-bit accuracy digitizer. Furthermore, the 8-bit accuracy is not adequate for several applications in which the observation of the small details in the image is crucial, since these details are weaker than 1 part in 256.
One known way to detect small changes in the image contrast is to apply image enhancement techniques, i.e., to increase the contrast of the image. Most of the currently available methods use various techniques to enhance the image after its initial digitization, using 8-bit accuracy. For example, there is available a video-enhancement system (mostly for biological observations) in which after the digitization, a background DC level is subtracted from the low contrast image, and then the difference is digitally amplified and displayed. Digital image processing is then used to enhance the image further, e.g., edge detection/enhancement, color coded display, etc. The common disadvantages of all of these approaches is that all the details that were smaller than 1 part in 256 gray levels were already lost, because they were digitally recorded with a precision of eight-bits only.
While digital cameras providing 14 bits accuracy exist, they are typically 100 times slower. Therefore, such cameras are not suitable for applications requiring a fast image acquisition/display rate.
Another known image enhancement method is applied to video data prior to the digitization. In such a case, applicable for low contrast images only, a background DC level is subtracted from the camera output, and the remaining level is amplified in an analog manner prior to the digitization. Such a DC subtraction is already implemented in several commercial cameras (internal subtraction) or in external devices which first subtract a DC level and than amplify the remaining signals, taking care of the synchronization of the video signal.